NATS supplies ATS to airports under two types of contract: fixed price such as Manchester, London City, Farnborough etc., and the BAA contracts. At BAA airports, NATS' revenue is derived from an Air Navigation Service Charge applied to each landing. This is based on the MTWA of each aircraft type and is charged per metric tonne or part thereof. At the london Airports the current rate is £1.19 per tonne for the first 100 tonnes, and £0.48 for each subsequent tonne or part thereof. At Edinburgh for example, the charge is £2.26 for each and every metric tonne. Revenue is therefore directly linked to the number of ATMs, BUT the trend for airlines to use smaller aircraft types when passenger demand is lower has a considerable impact on income for NATS. Here endeth the lesson. See AIP GEN 4.2.1 for more information.